The present invention relates to an ozone generator, wherein an anode and a cathode are closely adhered to each face of the fluororesin type cation exchange membrane, an electrode having conductive diamond on its surface is used as the anode, water is electrolyzed to evolve ozone from the anode and hydrogen from the cathode.
Ozone is known as a substance, in the nature, having an extremely strong oxidizing power and has been widening its applications over various industries. For instance, ozone is utilized in waterworks and sewage plants for the sterilizing and decolorizing treatments. Another advantage of ozone is the nature that it turns to harmless oxygen through autolysis with time lapse. Ozone, therefore, is appreciated as a chemical used for sterilizing and decolorizing processes which are easier and safer in handling than the former processes by chemicals, without secondary contamination by chemical residues or reaction by-products, offering an easy post-treatment.
Well known methods of ozone production include the UV lamp process, the silent discharge process, and the electrolysis process.
The UV lamp process produces ozone through exciting oxygen by UV rays, available from a relatively simple unit, but the production volume is small and the application area is limited to deodorizing rooms and cars.
The silent discharge process is one of the most prevalent and commonly used ozone generation methods. It is widely applied for various purposes ranging from simple room deodorizing by a small-scale ozonizer to industrial water treatment by a large-scale ozonizer with an output capacity of several tens kilograms per hour. The silent discharge process uses oxygen gas or oxygen in air as feed material, and generates ozone through excitation by electric discharge.
The electrolysis process generates ozone in the generated anodic gas through electrolysis of water. Ozone gas can be obtained also through the electrolysis of aqueous solutions including that of sulfuric acid; however, if the electrolysis is carried out with ultrapure water as raw material applying a fluororesin type cation exchange membrane, highly concentrated and purified ozone is obtained. The ozone production system by ultrapure water electrolysis has been widely used in the precision cleaning fields such as for semiconductor wafers or LCD substrates, since it applies ultrapure water as raw material and the generated gas contains impurities at an extremely low level.
Conventionally, lead dioxide (PbO2) deposited on the conductive porous metals including titanium by such means as electrolytic plating has been utilized as an anode for the electrolytic ozone process for its superior current efficiency in ozone gas generation. When ultrapure water is electrolyzed at a room temperature in a cell with perfluorosulfonic acid ion exchange membrane and the lead dioxide applied as the anode, the current efficiency in ozone generation is usually 10-15%, even as high as 20% at a high current density. Although perfluorosulfonic acid ion exchange membrane is consumed with time during electrolysis, the amount of consumption is small, and therefore, a constant ozone output and operation safety can be maintained in more than two consecutive years of electrolysis run.
As above-mentioned, the lead dioxide anode shows a high current efficiency in ozone generation under a high current density and in continuous electrolysis operation, as well as good long-term stability, but the lead dioxide anode is susceptible to be reduced and deteriorative in a reducing environment. For instance, in a suspension of electrolysis operation, the lead dioxide on the electrode surface is easily reduced to lead hydroxide (Pb(OH)2), lead oxide (PbO) or lead ion (Pb2+) through reactions with reducing materials including hydrogen remaining in the electrolytic cell or the electrolytic reducing reaction from cathodic polarization. Since these reduced substances have neither ozone generation ability nor electronic conductivity, such phenomenon is observed that the ozone generation capacity decreases in a resumed operation after suspension.
Therefore, the electrolytic ozone generator system applying lead dioxide electrodes usually has a mechanism to supply protective current in a range of 1/10- 1/1000 of electrolysis current normally applied to the electrolytic cell, to avoid performance deterioration during a cease of operation. Such mechanism, composed of a DC power source dedicated to the protective current supply, a battery, and a control system which constantly monitors state of the electrolytic cell so that no-current state does not occur even instantaneously. By this mechanism, lead dioxide anodes are protected from being exposed to a reducing environment even during the electrolysis operation cease; however, provision of such mechanism results in complicated working mechanism and configuration of the electrolytic ozone generator system, leading to a higher equipment costs.
Moreover, the lead dioxide anode contains a good amount of lead. Nowadays, use of lead tends to be reduced in all industrial commodities for its toxicity and legislative regulations, such as ROHS guide line. (Refer to Non-patent Document 1.)
On the other hand, it is known that the water electrolysis using conductive diamond as anode, where its conductivity has been induced by dopant such as boron added in the crystal structure, gives a current efficiency as high as 40% in terms of ozone output, which is far higher than the electrolysis using the lead dioxide anode. Moreover, the conductive diamond anode is superior in chemical and electrochemical stability and gives no change in property and electrolytic performance even in the reducing environment where lead dioxide changes in quality and degrades. Accordingly, the protective current mechanism, which is essential in the electrolytic ozone generator system using lead dioxide anodes, is not required, leading to a simple equipment design. As a matter of fact, neither carbon nor boron, constituting the conductive diamond is the object materials in the ROHS guide line.
However, it has been revealed that the conductive diamond electrode has a very strong oxidation capacity and therefore, if water electrolysis is conducted in the same manner as with the conventional electrolytic ozone generation cell, in which the conductive diamond electrode is made in contact with perfluorosulfonic acid ion exchange membrane, the consumption rate of perfluorosulfonic acid ion exchange membrane becomes more than 100 times compared with the case of the lead dioxide electrode. A rapid thinning of membrane by electrolysis operation leads to an abrupt increase in permeation of hydrogen gas evolved in the cathode compartment into the anode compartment, causing the concentration of hydrogen in the anode gas to excel the low limit of hydrogen explosion even in a short period of electrolysis, resulting in an electrolytic cell with an extremely short time of safety electrolytic operation. Accordingly, even if the conductive diamond electrode features excellent ozone generation capacity, commercial application in an electrolytic cell for ozone generator has been difficult.
Conventionally, in the ozone generation method in which an anode and a cathode are closely adhered to each face of the fluororesin type cation exchange membrane, an electrode having conductive diamond on its surface is used as the anode, water is electrolyzed to evolve ozone from the anode and hydrogen from the cathode, the consumption of fluororesin type cation exchange membrane is reportedly restricted by controlling electric current supply or by inclusion of reinforcing materials in the fluororesin type cation exchange membrane. (Refer to Patent Literature 1)
However, in this method, the current supply to the electrolytic cell is restricted to below the value at which the ozone generation efficiency reaches maximum, causing the controlling range of ozone output by the equipment in this electrolysis method to be narrow. Moreover, reinforcing materials contained in the fluororesin type cation exchange membrane will be exposed from the fluororesin type cation exchange membrane which has been consumed with time and made in contact with the conductive diamond electrode, causing electric current to be supplied no more at that time since reinforcing materials have no conductivity, resulting in no ozone output. In this case, the life span of the electrolytic cell is estimated to be the time up until the thickness from the surface of the fluororesin type cation exchange membrane to the reinforcing material has been consumed.